<div id="target" style="width: 100px; height: 100px; left: 50px; top: 50px;
                        background-color: blue; position:absolute;" />

<script>
target = document.getElementById('target');
document.onmousemove = evt => {
  // Do drag when the left mouse button is down.
  if (evt.buttons === 1) {
    target.style.left = (evt.clientX - 50) + 'px';
    target.style.top  = (evt.clientY - 50) + 'px';
    return false;
  }
}
</script>
